This invention relates to the fabrication of semiconductor devices and, more particularly, to a technique for achieving submicron control of lateral doping profiles in such devices.
It is known to fabricate microwave bipolar transistors using the so-called self-aligned-emitter process, as described, for example, in "Low-Noise Implanted-Base Microwave Transistors" by J. A. Archer, Solid State Electronics, Vol. 17, pp. 387-393, 1974. To maximize the signal-to-noise ratio of such a transistor, the total base resistance of the unit must be reduced to a minimum while at the same time maintaining a sufficient current gain. In such a device, the base typically comprises inactive and active regions. The total base resistance is the sum of the resistances of these two base regions. To keep the current gain constant the resistance of the active base region can be reduced only by reducing the width of the emitter stripe of the device. The resistance of the inactive base region is reduced by establishing a relatively high doping concentration in that region.
In a typical self-aligned-emitter transistor as originally proposed, the emitter and inactive base regions are directly adjacent to each other. If the inactive base region is relatively heavily doped, its adjacency to the emitter which is also heavily doped gives rise to a so-called soft emitter-to-base junction which, for many applications of practical interest, is an undesirable feature of the device. (A soft junction is one that exhibits a relatively high leakage current and a relatively low breakdown voltage.)
One proposal for improving the characteristics of the emitter-to-base junction of a self-aligned-emitter transistor is to include in the device a so-called buffer base region. (See, for example, "A New Submicron Emitter Formation with Reduced Base Resistance for Ultra High Speed Devices" by H. Kamioka et al., IEDM Technical Digest, pp. 279-282, 1974.) This buffer portion is sandwiched between the relatively lightly doped active base region and the relatively heavily doped inactive base region. The impurity concentration in the buffer portion is typically intermediate those of the active and inactive base regions. Since the width of the buffer portion is typically very small (for example, less than 0.3 microns) its inclusion in the device does not contribute significantly to the total base resistance.
The technique described in the aforecited Kamioka et al. article for achieving a buffer base region in a self-aligned-emitter transistor has the disadvantage that it requires the controlled chemical etching of four deposited layers. In addition, an undercutting step of the type specified by Kamioka et al. has been found to be difficult to control in a predictable and reproducible way.
Accordingly, the need arose for a technique that would permit the formation of a buffer base region in a self-aligned-emitter transistor in a simple but easily reproducible manner. Moreover, it was recognized that such a technique if available would also be useful to control lateral doping profiles in a variety of other semiconductor devices.